1. Field of the Invention
The present invention relates to a gradient coil arrangement for a nuclear magnetic resonance tomography apparatus, in which a basic magnetic field is produced by ring coils that enclose an examination chamber and whose coil axes fie along a longitudinal axis of the examination chamber, wherein all the effective segments of the gradient coils are arranged respectively on two radially spaced surfaces that lie symmetrical to the longitudinal axis of the examination chamber, so that the gradient coil arrangement remains open laterally.
2. Description of the Prior Art
A gradient coil arrangement of this type is known, for example, from the article by M. A. Martens et al., "Insertable biplanar gradient coils for magnetic resonance imaging," Review of Scientific Instrumentation 62(11), November 1991, pages 2639 to 2645. This article relates to coils known as "local" gradient coil arrangements, which typically are not installed fixedly in the MR apparatus, but rather are inserted only given the examination of particular body parts (e.g. the head). Since such local gradient coil arrangements can enclose a smaller examination volume than fixedly installed gradient coil arrangements, the magnetic energy stored therein becomes correspondingly smaller. It is thereby possible to achieve correspondingly shorter rise and fall times of the gradient pulses, with the same demands on the gradient coils and the gradient amplifier being operated the same as for a fixed coil. Local gradient coils are thus suited in particular for pulse sequences in which gradients have to be switched very rapidly, e.g. for the EPI (echoplanar imaging) method.
In the gradient coil arrangement described in the above article, this arrangement is deployed on two parallel plates containing all the gradient coils required for the gradient field production in three directions. The plate-shaped structures can be brought into close proximity to the subject under examination, so that the magnetic energy stored essentially between these plates remains lower than in conventional whole-body arrangements.
Conventional, fixedly installed gradient coil arrangements occupy a considerable portion of the interior space of a magnet arrangement (called the "warm bore" in superconducting magnets). This is explained in more detail with the help of FIG. 1. FIG. 1 shows a highly schematic representation of the components of a nuclear magnetic resonance tomograph. The basic magnetic field is produced with a magnet 1 in a ring coil arrangement that is constructed in superconducting fashion. In superconducting basic field magnets, the coils (not shown in FIG. 1) are arranged in a cryostatic temperature regulator. The basic field magnet has a hollow cylindrical interior space. A gradient coil arrangement 2 with a hollow cylindrical shape is arranged in this interior space. A radio-frequency antenna 3 is provided inside the gradient coil arrangement 2. The interior space remaining after the installation of the gradient coil arrangement 2 and the RF antenna 3, and after the assembly of sheathings (not shown), is available as useful space. A patient 10 can be placed in this useful space on a patient table 4. For the useful space, certain minimum dimensions are required, particularly with respect to the width, in order to enable the examination of patients in general with a certain degree of comfort in their positioning, and to enable overweight patients to be examined at all. The required inner diameter of the basic field magnet is thus given by the desired dimensions of the useful space, as well as by the radial extension of the RF antenna 3 and the gradient coil arrangement 2. The inner diameter of the basic field magnet 1, however, determines its cost. It is necessary to construct not only the ring coils, and also the cryostatic temperature regulator in superconducting systems, with a larger diameter. It is also necessary to apply more magnetic energy at a given magnetic field strength, due to the larger interior volume. Given constant preconditions relating to the required homogeneity in the examination chamber, with a larger inner diameter of the basic field magnet the length thereof must also be increased. This is not only highly undesirable from the point of view of cost, but also causes increased claustrophobia problems in the patients, and worsens accessibility to the patient.
Of the systems installed in the examination chamber of the basic field magnet 1, the gradient coil arrangement has the largest space requirement.
For the explanation of the basic problem to which the present invention is directed, in FIG. 2 a conventional gradient coil system for producing a magnetic field gradient in the y-direction is schematically shown. In nuclear magnetic tomography apparatuses, magnetic field gradients in three directions (x, y and z) perpendicular to one another, corresponding to the coordinate system shown in FIG. 2, are required. The direction of the basic magnetic field B.sub.z, i.e. the longitudinal axis of the hollow cylindrical examination chamber, is defined as the z-direction. FIG. 2 shows only the gradient coil system of conventional construction used to produce a magnetic field gradient in the y-direction. This gradient coil system consists of four individual saddle coils 5 to 8. The inner curves 5a to 8a of the saddle coils essentially contribute to the production of the magnetic field gradient in the y-direction, called the y-gradient for short in the following; the outer curves 5b to 8b lie at a greater distance from the spherical examination region 9, for which both the homogeneity requirements concerning the basic magnetic field and linearity requirements concerning the gradient fields must be maintained. The effect of the inner curves 5a to 8a on the magnetic field in the spherical region of examination 9 is indicated in FIG. 2 with arrows. In the upper part of the region of examination 9, an amplification of the basic magnetic field B.sub.z is achieved, and in the lower region an attenuation of this field is achieved, so that a magnetic field gradient in the y-direction arises. For the production of a magnetic field gradient in the x-direction, the same coil arrangement is again present, but rotated 90.degree. about the cylinder axis, but is not shown in FIG. 2 for clarity. Finally, for the production of a magnetic field gradient in the z-direction, ring coils (also omitted in FIG. 2 for clarity) are arranged on the cylindrical support (carrier).
It should be noted that more modern gradient coil arrangements are no longer constructed from simple segment curves. Rather, more complex conductor structures, resembling a fingerprint, arise as a result of optimization methods, as described for example in U.S. Pat. No. 5,309,107. This has no effect, however, on the basic problem described herein.
Even when the individual gradient coils are of relatively flat construction, a non-negligible thickness of the overall gradient coil arrangement results from the necessity of stacking three coil structures over one another. This is particularly so when gradient shielding coils are present that are intended to prevent the occurrence of eddy currents in the metallic outer wall of the examination chamber 1c by outwardly compensating gradient fields. Gradient coils of this sort, called "actively shielded," are shown for example in German OS 44 22 782. Moreover, the distance between the useful portion of the gradient coil and the gradient shielding coil is reduced outwardly in order to reduce the parasitic flux density. In an embodiment described in this publication, the available examination chamber is thereby further expanded outwardly by expansion (widening) of the gradient useful coils.
From German OS 195 04 171, a local gradient coil arrangement for a nuclear magnetic resonance tomography apparatus is described that has a substantially hollow cylindrical geometry, as do conventional gradient coils. In order to enable simpler application of the local gradient coil arrangement to an examination subject, it can be separated at least on one side along a separation line that runs in the axial direction. The coil design itself corresponds in principle to the conventional gradient coil, but all the gradient coil conductors that cross the separating line are interrupted at the separating line. On each side of the separating line, the currents are conducted via connection leads that run parallel to the line of separation.
Finally, from European Application 0 313 213 a gradient coil arrangement is known in which one of the gradient coils, namely the y-gradient coil, lies closer to the examination subject than do the other gradient coils. A stronger gradient field thus can be produced in the y-direction, with otherwise identical preconditions.